The present invention relates to a digital communication system in which the initial bit rate is reduced for transmission in a transmission path at a transmitter that includes an encoder which converts a digital input word sequence into a line code with a reduced bit rate while a receiver arranged at the other end of the transmission path includes a decoder that converts the line code into a sequence of digital output words at the original bit rate.
Communication systems of this kind are disclosed in various references, e.g., in German Pat. No. 23 55 676. The encoder and the decoder used there are a DPCM encoder and a DPCM decoder, respectively (DPCM=differential pulse-code modulation).
In a DPCM encoder, a reduction of the bit rate to one half, which may be necessary for television picture transmission because of the predetermined sampling frequency and the predetermined maximum bit rate on the transmission path, is possible only by converting the input-word sequence, applied at the sampling rate with a word length of 8 bits, into an output-word sequence of the same word-repetition rate but with the word length halved, i.e., with 4 bits per word. However, this encoding results in a visible loss of quality.